Motor-driven lenses for zooming and/or focusing have been designed for photographic cameras, video cameras, electronic still cameras, and the like. In one arrangement, a motor is used to drive a worm screw on which is mounted a lens holder, resulting in bidirectional movement of the lens. This structure is relatively large, bulky, heavy and complex; and typically is used to provide gross movements (as opposed to fine movements) of the lens. Consequently, apparatus of this type has been used heretofore to implement a motor-driven zoom operation, but typically not a motor-driven focusing operation.
Other motor-drive arrangements which can be used both for zooming and focusing employ what has come to be known as a voice coil motor. One example of a motor-driven lens assembly which incorporates a voice coil motor is described in U.S. Pat. No. 5,220,461, assigned to the assignee of the present invention. Another example of a motor-driven lens assembly which incorporates a voice coil motor is illustrated in accompanying FIGS. 1 and 2. This assembly finds particular application in the lens barrel of a video camera, such as the type shown in FIG. 3. In the lens assembly of FIG. 1, a fixed lens, referred to as a forward lens unit 120, cooperates with a zoom lens 121 which is driven by a stepping motor 122. An image from zoom lens 121 passes through a diaphragm unit 123 to a focusing lens unit 125. The combination of zoom lens 121 and diaphragm 123 typically is referred to as a zoom unit 126. Focusing unit 125 is comprised of a fixed lens 124, a movable unit 103 which includes a lens whose position is adjusted to effect proper focusing and a fixed unit 102 supported within a casing 101. Units 102 and 103 in combination comprise a voice coil motor, as will be described. Movable unit 103 includes bearing members slidable on shafts 104a and 104b which are mounted in casing 101 and extend in a direction parallel to the longitudinal axis of the voice coil motor.
Stepping motor 122 is used to vary the position of zoom lens 121 over a relatively wide range so as to effect a zoom function to provide both telephoto and wide-angle imaging. The range of movement of unit 103 is relatively small in comparison to that of stepping motor 122, thus facilitating the use of a voice coil motor to effect focusing adjustments.
As illustrated more particularly in FIG. 2, fixed unit 102 of the voice coil motor is disposed in casing 101 and constitutes a stator structure. Movable unit 103 mates with the stator structure an is slidable on shafts 104a and 104b in the axial direction (i.e. along the longitudinal axis) of this structure. As illustrated, the stator structure is comprised of two sets of frame plates 109 and 110 which are referred to herein as outer and inner frame plates, respectively. Inner frame plate 110 is configured as a substantially rectangular tube and outer frame plates 109 are formed of rectangular plate members spaced from and parallel to the walls of the inner frame plates. A rectangular connecting plate 108 has outer frame plates 109 extending therefrom, these outer frame plates being bent by 90.degree. and, thus, are perpendicular to connecting plate 108. Inner frame plates 110 are secured to connecting plate 108 such that a magnetic flux path is provided from, for example, outer frame plate 109 through connecting plate 108 to inner frame plate 110. The connecting plate is provided with a central opening 107 of rectangular shape concentric with the interior of the rectangular tube defined by inner frame plates 110. Preferably, a rectangular-shaped magnet 106 is secured to each of outer frame plates 109. In the illustrated embodiment, the surface of an outer frame plate 109 which faces the surface of an inner frame plate 110 has magnet 106 secured thereto, as by a suitable cement. Stated otherwise, a magnet 106 is secured to an inner surface of an outer frame plate 109 and is spaced from an outer surface of an opposite inner frame plate 110 so as to define a gap between the magnet and the inner frame plate. Each of magnets 106 is polarized in the direction of its thickness so that magnetic flux traverses the path from the magnet to inner frame plate 110 through connecting plate 108 to outer frame plate 109 and then to magnet 106.
Movable unit 103 is formed of a lens holding frame 112 to which is secured a bobbin 113 having a coil 118 wound thereon so as to extend in the axial direction, as illustrated. The lens holding frame may be of metal to which a frame support 114 is secured. The frame support and lens holding frame may be of integral construction or may be formed as separate pieces suitably bonded together. Frame support 114 is a rectangular plate having a center opening which is at least coextensive with the diameter of a lens 111 mounted on and supported by a cylindrical tube 115 which, in turn, is supported by frame support 114. A pair of bearings 116a and 116b is secured to tube 115, the bearings extending radially outward to receive shafts 104a and 104b, respectively. The combination of frame support 114 and tube 115 constitute lens holding frame 112 which is slidable on shafts 104a and 104b. Since bobbin 113 is secured to lens holding frame 112, the bobbin also is slidably movable in the axial direction of the illustrated assembly. Bearings 116a and 116b are provided with through-holes 117a, 117b, respectively, through which pass shafts 104a and 104b.
Bobbin 113 is formed of a synthetic resin and is shaped as a substantially rectangular tube having a rectangular central hollow section herein. The bobbin is formed with a peripheral winding slot in which coil 118 is wound. It is appreciated that the rectangular tube-shaped bobbin fits within the gap between magnets 106 and inner frame plates 110 of the stator structure such that the coil passes through the magnetic flux which is generated in the gap by magnets 106. Thus, inner frame plates 110 fit within the central hollow section of bobbin 113 and the bobbin itself fits within the gap between the inner frame plates and magnets 106.
Coil 118 is coupled to suitable connectors (not shown) so that driving current may be supplied thereto. Depending upon the direction of this current, a force proportional to the magnitude of the current and the magnetic flux from magnets 106 is exerted on the coil, thereby sliding coil 118, bobbin 113 and lens holding frame 112 in the corresponding direction. That is, as a result of the current supplied to the coil, movable unit 103 is driven axially relative to fixed unit 102.
However, the limits of the bidirectional axial movement of movable unit 103 are constrained by the abutment of the free end of bobbin 113 against connecting plate 108 and also the abutment of the free, or forward, end of inner frame plates 110 against frame support 114. It is important that coil 118 always is positioned within the gap between magnets 106 and inner frame plates 110 and does not extend forwardly thereof. Consequently, axial movement of movable unit 103 is constrained by the axial length h of frame support 114. To permit a larger range of movement and, thus, a greater degree of focus adjustment, the axial length h must be increased or, alternatively, the axial length of bobbin 113 must be increased such that the coil extends deeper into the gap between magnets 106 and inner plates 110. Of course, to provide greater axial movement of movable unit 103, it is necessary to increase the axial length of fixed unit 102. Hence, the overall length of the motor-driven lens assembly using the voice coil motor arrangement shown in FIG. 2 and, thus, the overall axial length of the lens barrel in which this arrangement is disposed, becomes greater if the range of adjustment is to be increased. As a result, the weight of the adjustable lens assembly likewise is increased.
Moreover, in the arrangement shown in FIG. 2, the free or forward ends of outer frame plates 109 and inner frame plates 110 must remain opened to permit coil 118 to move within the gap therebetween. That is, the forward end of the gap cannot be closed. Consequently, it is not possible for the free or forward ends of the inner and outer plates to be magnetically connected in a low reluctance magnetic path, such as the low reluctance path provided by a plate similar to connecting plate 108. Accordingly, the magnetic flux path within the stator structure is unidirectional, that is, it extends in only one direction in each of outer plates 109 and likewise extends in only one direction in each of inner plates 110, thus reducing the magnetic efficiency of the stator structure. To improve this magnetic efficiency, the inner and/or outer plates must be made thicker, resulting in an overall arrangement that is larger in size and greater in weight.